22 The Living Plant 



shading), so nearly transparent as to be almost invisible. But 

 though so insignificant in appearance, this grayish material 

 is nevertheless the most important of all substances, for it is Proto- 

 plasm, the exclusive seat and sole physical basis of all the phe- 

 nomena of life, as I shall show in a later chapter devoted to that 

 subject. Within this living substance, close up to the wall, lie 

 the chlorophyll grains, each of which has a definite shape, some- 

 thing like that of a disc or a lens, and consists of denser proto- 

 plasm deeply stained by a green liquid which is the chlorophyll 

 substance proper. Finally, it should be added, in order to com- 

 plete the reader's conception of the cell, that all of the remainder 

 of its interior is filled with the sap, which is simply water contain- 

 ing many kinds of substances in solution. As to the spaces be- 

 tween the cells, they contain as a rule nothing but air, which is 

 in connection with the atmosphere outside of the plant through 

 tiny little openings, called stomata, between the cells of the 

 epidermis. We shall return, and that often, to this subject of 

 cellular structure, and the reader will then recognize the ad- 

 vantage of having thus made some preliminary acquaintance 

 therewith. 



We must now return to the problem involved in the observa- 

 tion that a close connection exists between the distribution of 

 chlorophyll and the presence of light. Observation alone, how- 

 ever, cannot lead any farther, and we must resort to the second 

 of the biologist's methods, experiment. In such a situation 

 the scientific mind would reason somewhat like this, if, as 

 seems implied by the facts, the chlorophyll has in the plant a 

 function dependent on the action of light, then some difference 

 should develop between leaves kept for a tune in darkness and 

 others kept equally long in light. Accordingly the experimenter 

 would darken certain leaves on a plant, in a way that would not 

 injure their health, and then, after a day or two, would examine 

 a darkened and lighted leaf side by side. The result is always 

 disappointing to the naked eye, by which no differences at all 



